A wireless transmitter translates modulated signals into a format suitable for radio transmission, shifts the carrier to microwave frequencies, and adjusts the output power level.
The transmit power level is especially important in self-interfering systems, such as in code division multiple access (CDMA) systems. This is because other users that share the same frequency channel are treated as noise. Consequently, power control is essential, with the output level rarely reaching its peak and typically following a distribution like the one shown in FIG. 1.
Most wireless transmitters require power amplifiers to reach peak power levels. The power amplifier may have to produce up to 1 watt of power without adding noise, with minimal spectral regrowth and little sensitivity to large standing waves caused by antenna reflections. Due to these design constraints, the power amplifier is invariably optimized for peak power performance and is generally less efficient at lower power levels. This is important because the power amplifier consumes more power than any other circuit in a communication system.
It would therefore be advantageous to have a system that improves the power efficiency of the transmitter and power amplifier at all output power levels.